DOCTORS may one day be able to use X-rays to destroy the blood vessels that feed a brain tumour—without the debilitating side effects of current treatments. The trick is to blast the tumour with more than 20 ultrathin X-ray beams, instead of the single broad beam of conventional radiotherapy
Although these “microbeams” will also puncture healthy blood vessels, only those that feed the tumour become leaky and die. This is because cancer cells do not communicate with each other like healthy capillary cells, so cannot repair the gaps created by X-rays. (see Graphic). While the cancer cells die, the healthy cells repair themselves, Avraham Dilmanian of the Brookhaven National Laboratory in New York state told the meeting.
Dilmanian first showed about a year ago that microbeam radiation therapy slows tumour growth and causes fewer side effects in rats. In MRT, 100-micrometre-wide beams cut through cells lining a capillary and kill them, but the cells in the gaps between beams are spared. To find out why only healthy capillaries survive the treatment, Dilmanian used light emitters to tag the blood flow in rats. He found that eight hours after exposure to the microbeams, the capillaries surrounding tumours leaked blood, while those in healthy tissue did not.
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As healthy cells constantly communicate, Dilmanian believes that when a cell is damaged, the healthy cells either side push it out and then elongate to bridge the gap. But communications are very different between tumour cells. Their membranes are abnormal and not capable of responding to their neighbours in this way. So the vessels leak and the tumours they feed die.
MRT could drastically reduce conventional radiation therapy’s side effects, such as severe nausea and hair loss, which are often caused by damage to healthy cells. In addition, children under the age of three might be able to receive MRT, whereas current radiation therapy kills too many normal cells to be safe for delicate, young brains.
Another bonus is that MRT is a one-off treatment. This is partly because the microbeams do not damage normal tissue, so there is no need to spread the dose out over weeks, but also because the human brain floats freely in the skull and pulsates in time with the heartbeat. If the microbeams were applied for any length of time, the vital gaps between them would smear.
So Dilmanian only applies microbeams for milliseconds, using X-rays from a synchrotron – a machine used in research to deliver a powerful dose of X-rays in a short space of time. The X-rays are funnelled towards the patient through a magnetic tube and split into slices by a tungsten mask in front of the synchrotron beam.
Using X-rays to destroy the blood vessels that feed tumours is similar in concept to “anti-angiogenesis” drugs, which doctors have long hoped will be a viable therapy. “We are not in the business of killing tumour cells, we deprive them of food and oxygen and they die of starvation and possibly other associated effects,” explains Dilmanian.
He says the technique may be safe to test in people in three years’ time. For routine treatment, though, you would have to build a synchrotron specially designed for MRT – a costly option. Edward Hoffman from the David Geffen School of Medicine in Los Angeles believes that the technique will only catch on if it can be made to work without a synchrotron.
MRT could also be used to clear away diseased nerve cells characteristic of multiple sclerosis. This might make room for new neurons grown using future stem cell technologies.